Metamaterial surface antenna technology (MSAT) is a recent advancement in reconfigurable antenna technology. Metamaterial surface antennas, also known as surface scattering antennas or metasurface antennas, are described, for example, in U.S. Patent Application Publication No. 2012/0194399 (hereinafter “Bily I”). Surface scattering antennas that include a waveguide coupled to a plurality of subwavelength patch elements are described in U.S. Patent Application Publication No. 2014/0266946 (hereinafter “Bily II”). Surface scattering antennas that include a waveguide coupled to adjustable scattering elements loaded with lumped devices are described in U.S. Application Publication No. 2015/0318618 (hereinafter “Chen I”). Surface scattering antennas that feature a curved surface are described in U.S. Patent Application Publication No. 2015/0318620 (hereinafter “Black I”). Surface scattering antennas that include a waveguide coupled to a plurality of adjustably-loaded slots are described in U.S. Patent Application Publication No. 2015/0380828 (hereinafter “Black II”). And various holographic modulation pattern approaches for surface scattering antennas are described in U.S. Patent Application Publication No. 2015/0372389 (hereinafter “Chen II”).
While the above applications are principally focused on waveguide embodiments, where a reference wave or feed wave is delivered to the adjustable scattering elements via a waveguide underlying the scattering elements, in other approaches, the reference wave or feed wave may be a free-space wave that is delivered to the adjustable scattering elements by illuminating from above a reflective surface that is populated with adjustable scattering elements. An example is shown in U.S. Patent Application Publication No. 2015/0162658 (hereinafter “Bowers”). Embodiments of the present invention use a similar free-space feed configuration, with a reflective surface that is illuminated from above, the reflective surface being populated with adjustable scattering elements. It will be appreciated that, throughout this disclosure, whenever an embodiment is disclosed that involves illuminating a surface with a free-space reference wave to provide a reflected outgoing (transmit) wave having a specific field pattern, a reciprocal embodiment is also contemplated that involves reflecting a incoming (receive) wave from the surface and then detecting the reflected wave according to the same specific field pattern.
Furthermore, while the above applications are principally focused on embodiments operable at radio or microwave frequencies, the present invention involves embodiments operable at higher frequencies, especially at infrared or visible frequencies. When operating frequencies are scaled up to optical (infrared/visible) frequencies, the sizes of the individual scattering elements and the spacings between adjacent scattering elements are proportionally scaled down, to preserve the subwavelength/metamaterial aspect of the technology. The relevant length scales for operation at optical frequencies, typically on the order of microns or smaller, are smaller than the typical length scales for conventional PCB processes, so embodiments of the present invention may instead be implemented using micro- and nano-lithographic processes such as CMOS lithography, PECVD, and reactive ion etching.
Finally, while the above applications typically use metallic structures with resonances that are adjustable to provide the adjustable element responses, these metallic structures become increasingly lossy as the operating frequency approaches optical frequencies. These losses are undesirable because they degrade the efficiency of a reconfigurable antenna that is implemented using metallic resonant structures. Embodiments of the present invention mitigate the lossy characteristics of metals at optical frequencies by using primarily dielectric structures having primarily dielectric resonances.
Various applications of an optical surface scattering antenna as described herein include, but are not limited to: imaging via LIDAR; imaging via structured illumination; free-space optical communication, either single-beam or MIMO; and pointing and tracking for free space optical communications.